‘Build tight, ventilate right’. For over a decade now, this mantra has been the rallying cry of energy-efficiency experts but, until recently, it seemed to be landing on deaf ears.

To many in the UK, the concept of ‘airtightness’ seems somehow alien and unwelcome. ‘I don’t want to live in a plastic bag’ being a typical response. Or ‘I always sleep with the windows open, so what’s the point?’ They fear that an airtight house will be an unhealthy one, full of stale smells and condensation. Indeed, the classic Victorian house was designed to be as air-leaky as possible in order to expel the coal smoke: with rattling sash windows, draughts through the floorboards and open fireplaces. This, we thought, was healthy. But perceptions are changing. Now, with the growing awareness of concepts like PassivHaus and zero carbon homes, people are cottoning on to just how important airtight­ness is in low-energy houses.

In an airtight house, air quality isn’t down to random draughts, it has to be planned: the ‘ventilate right’ bit of the equation. Airtight homes rely on whole-house ventilation systems. We thus have three essential elements to a low-energy home: lots of insulation; airtightness; and a whole-house ventilation system. The first won’t work without the second; the second won’t work without the third; and if you combine all three, you get a low-energy, comfortable home with good indoor air quality.

Testing Airtightness

Airtightness is measurable. To test for it, you remove the front door and replace it – temporarily – with a blower door which pumps air into the house, putting it under a small pressure load of 50 Pascals. This measures how much air is needed to be pumped in to keep the house at a steady 50 Pascals. The air leakage measurements are scored in cubic metres of air moving every hour through a square metre of the building envelope under 50 Pascals of pressure, often shortened to just q50. It doesn’t relate to what happens in practice because homes aren’t under pressure like this, but the figure does give a useful way of comparing how leaky a house is. And, in practice, it also tells us how well a house has been designed and built. It’s a sort of quality mark.

The Building Regulations in England and Wales state that an airtightness pass score is 10 q50. That’s not very difficult to beat, and a pass score of 10 q50 could in no way be described as an ‘airtight house’. There is, in any event, a dispensation for sites with just one or two homes. Here, you can forgo the £300 or so air pressure test but you will have to assume a score of 15 q50, which will upset your Dwelling Emission Rate calculations and require you to install better insulation and other energy efficiency measures to offset heat losses. It may well be cheaper to have a test done and, if you are aiming for an ‘airtight house’, then the test really is essential. In the rare event of a test failure, remedial steps can often be undertaken at the time of the test.

10 q50 is not a high standard and can generally be easily met by careful draught-proofing and the application of mastic here and there. A score below 3 q50 is reckoned to be pretty good, and this is the level where mechanical ventilation starts to become a necessity. The gold standard of low-energy housebuilding – PassivHaus – demands an airtightness score of just 0.6 q50.

Designing it in

The key to getting a good score in an airtightness test is to design in an air barrier at the outset, and then to ensure that it is properly installed, and not tampered with during construction. For instance, instead of allowing plumbers to drill holes wherever they like, the service connections need to be planned in advance and the air barrier penetrations sealed accordingly.

Tapes and Membranes

Airtightness tapes and membranes offer a simple way of achieving high levels of building airtightness. They are flexible and highly adhesive for sealing circular pipework or ductwork penetrations, the corners of window and door frames, the abutment between OSB or wood-fibre insulation boards or sheets of vapour barrier. NBT’s range of Siga tapes (above) are free from toxins, solvents, plasticisers and formaldehyde, and will maintain their peak performance for over 60 years, allowing for building movement over time. natural-building.co.uk

Heating an Airtight House

The good news is that the better insulated and more airtight a house, the less heating it needs, as it is able to hold onto heat generated by day-to-day activities of the occupants. But building airtight is bad news for lovers of traditional open fires — they allow far too much air leakage to be a viable option. Woodburning stoves with an attachment which lets you draw air from outside are popular; however, they are still not truly room sealed, and every time you open the door to re-fuel, you upset the airflow balance and release some smoke into the room. However, as an accompaniment to underfloor heating on just the coldest days, it’s not a bad solution. Being as airtight houses rely on whole-house ventilation anyway, the introduction of heat recovery to a mechanical system which controls air changes is a popular strategy.

Mechanical Ventilation

How Does it Work?
MVHR continuously extracts polluted air from a building. The stale air is passed through a heat exchanger, which is then used to pre-warm incoming fresh air. This ensures a fresher, warmer and more comfortable environment.

What are the Key Benefits?
Airtight properties cause a rise in internal humidity and increase the damage that can be caused through condensation to both the property and the health of the people living in it. MVHR provides a better quality of clean, fresh and healthy air. It also impacts on energy bills — recovering up to 95% of otherwise wasted heat, ultimately reducing the overall heating requirement. Also, no window trickle ventilation is needed, avoiding noise ingress.

What Does it Cost?
From £2-5,000, largely depending on the house size.

How is it Installed?
A certain amount of ducting is required in order to supply every room, but the unit itself is mounted to the wall on brackets.?The condense drain must be connected to the house’s wastewater system, and the unit is connected to the electrical supply. For a step-by-step DIY guide to installing a MVHR system click here.

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